1. Field of the Invention
The present invention generally relates to computer apparatus and, in a preferred embodiment thereof, more particularly relates to heat dissipation apparatus for computer microprocessors.
2. Description of Related Art
As computer microprocessors are provided with faster and faster clock speeds, the operating heat which these devices generate, and must be appropriately dissipated, correspondingly increases. The typical microprocessor used in a computer has a die portion (the microprocessor proper) which is exposed within a recessed area on the top side wall of the processor housing module (or "can") which is mounted on the computer's system board.
The operating heat from this type of recessed processor die has heretofore been dissipated using a die cast metal heat sink screwed down to the top side of the processor housing module over the recessed die. In order to provide the requisite heat transfer interface between the bottom side of the heat sink and the top side of the recessed die it was necessary to place a layer of thermal grease between the top side of the die and the underside of the metal heat sink. As is well known, while thermal grease provides a good heat conduction path between the die and the heat sink, it is considered to be an undesirable interface material from a manufacturing standpoint because it tends to be quite messy and is easily spread beyond its originally intended location.
A potentially better interface material is a compliant thermal interface pad compressed between the bottom side of the heat sink and the top side of the die. However, in practice the use of such a compliant thermal interface pad in lieu of thermal grease has not been practical with current high speed microprocessors of this general type due to two design criteria--namely, bond line thickness and die pressure.
Bond line thickness refers to the variable height between the top side of the recessed die and the top side of the processor housing, the variation in the bond line thickness arising due to manufacturing tolerances. TO compensate for this dimensional variance in bond line thickness, and to provide the necessary compression of a compliant thermal interface pad, the thermal interface pad must have a thickness that undesirably reduces the amount of die operating heat conducted therethrough. Specifically, the pad must have an initial undeformed thickness that extends from the top side surface of the recessed die to above the top side of the processor housing. Additionally, at least one manufacturer of high speed microprocessors of this type is now specifying a maximum pressure which may be exerted on the die. Thus, particularly when the bond line thickness is at the low end of its manufacturing tolerance level, the compression of a thermal interface pad against the die by the overlying, rigidly attached die cast heat sink can easily exceed this maximum design pressure.
Thus, the use of thermal grease as the heat transfer interface between the heat sink structure and the top side of the die has been the only technique that compensates for bond line thickness variations, while at the same time preventing excess pressure from being exerted by the heat dissipation apparatus on the recessed die and providing a satisfactory heat conduction path between the die and the heat sink secured to the top side of the processor housing. The bond line thickness variation from processor to processor, of course, makes it quite difficult to apply the correct amount of thermal grease to ensure that an efficient thermal interface is provided between the die and the overlying heat sink, while at the same time avoiding the attendant mess of placing too much grease in the die recess. As a practical matter, the amount of grease must be that which generally corresponds to the lowest die level within the housing recess (as determined by the manufacturer's bond line thickness tolerance range). Thus, when the top side of the die is at the low end of the bond line thickness tolerance range (i.e., at its highest permissible point within the die recess), the applied quantity of thermal grease is more than is needed. This undesirably aggravates the tendency of thermal grease to "migrate" and generally create a mess.
As can readily be seen from the foregoing, a need exists for an improved technique for dissipating operating heat from a recessed microprocessor die, of the type generally described above, without the previous necessity of using thermal grease as the heat transfer interface between the top side surface of the die and the heat dissipation apparatus overlying the die. It is to this need that the present invention is directed.